Categories C76-C80 include malignant neoplasms for which there is no clear indication of the original site of the cancer or the cancer is stated to be "disseminated", "scattered" or "spread" without mention of the primary site. In both cases the primary site is considered to be unknown.

2. Functional activity

All neoplasms are classified in this chapter, whether they are functionally active or not. An additional code from Chapter IV may be used, if desired, to identify functional activity associated with any neoplasm. For example, catecholamine-producing malignant phaeochromocytoma of adrenal gland should be coded to C74 with additional code E27.5; basophil adenoma of pituitary gland with Cushing's syndrome should be coded to D35.2 with additional code E24.0.

3. Morphology

There are a number of major morphological (histological) groups ofmalignant neoplasms: carcinomas including squamous (cell) and adeno-carcinomas; sarcomas; other soft tissue tumours including mesotheliomas; lymphomas (Hodgkin's and non-Hodgkin's); leukaemia; other specified and site-specific types; and unspecified cancers. Cancer is a generic term and may be used for any of the above groups, although it is rarely applied to the malignant neoplasms of lymphatic, haematopoietic and related tissue. "Carcinoma" is sometimes used incorrectly as a synonym for "cancer".

In Chapter II neoplasms are classified predominantly by site within broad groupings for behaviour. In a few exceptional cases morphology is indicated in the category and subcategory titles.

For those wishing to identify the histological type of neoplasm, comprehensive separate morphology codes are provided ( see section Morphology of neoplasms). These morphology codes are derived from the second edition of International Classification of Diseases for Oncology (ICD-O), which is a dual-axis classification providing independent coding systems for topography and morphology. Morphology codes have six digits: the first four digits identify the histological type; the fifth digit is the behaviour code (malignant primary, malignant secondary (metastatic), in situ, benign, uncertain whether malignant or benign); and the sixth digit is a grading code (differentiation) for solid tumours, and is also used as a special code for lymphomas and leukaemias.

4. Use of subcategories in Chapter II

Attention is drawn to the special use of subcategory .8 in this chapter [see note 5]. Where it has been necessary to provide subcategories for "other", these have generally been designated as subcategory .7.

5. Malignant neoplasms overlapping site boundaries and the use of sub- category .8 (overlapping lesion)

Categories C00-C75 classify primary malignant neoplasms according to their point of origin. Many three-character categories are further divided into named parts or subcategories of the organ in question. A neoplasm that overlaps two or more contiguous sites within a three-character category and whose point of origin cannot be determined should be classified to the subcategory .8 ("overlapping lesion"), unless the combination is specifically indexed elsewhere. For example, carcinoma of oesophagus and stomach is specifically indexed to C16.0 (cardia), while carcinoma of the tip and ventral surface of the tongue should be assigned to C02.8. On the other hand, carcinoma of the tip of the tongue extending to involve the ventral surface should be coded to C02.1 as the point of origin, the tip, is known. "Over-lapping" implies that the sites involved are contiguous (next to each other). Numerically consecutive subcategories are frequently anatomically contiguous, but this is not invariably so (e.g. bladder C67.-) and the coder may need to consult anatomical texts to determine the topographical relationships.

Sometimes a neoplasm overlaps the boundaries of three-character categories within certain systems. To take care of this the following subcategories have been designated:

An example of this is a carcinoma of the stomach and small intestine, which should be coded to C26.8 (Overlapping lesion of digestive system).

6. Malignant neoplasms of ectopic tissue

Malignant neoplasms of ectopic tissue are to be coded to the site mentioned, e.g. ectopic pancreatic malignant neoplasms are coded to pancreas, unspecified (C25.9).

7. Use of the Alphabetical Index in coding neoplasms

In addition to site, morphology and behaviour must also be taken into consideration when coding neoplasms, and reference should always be made first to the Alphabetical Index entry for the morphological description.

The introductory pages of Volume 3 include general instructions about the correct use of the Alphabetical Index. The specific instructions and examples pertaining to neoplasms should be consulted to ensure correct use of the categories and subcategories in Chapter II.

8. Use of the second edition of International Classification of Diseases for Oncology (ICD-O)

For certain morphological types, Chapter II provides a rather restricted topographical classification, or none at all. The topography codes of ICD-O use for all neoplasms essentially the same three- and four-character categories that Chapter II uses for malignant neoplasms (C00-C77, C80), thus providing increased specificity of site for other neoplasms (malignant secondary (metastatic), benign, in situ and uncertain or unknown).

It is therefore recommended that agencies interested in identifying both the site and morphology of tumours, e.g. cancer registries, cancer hospitals, pathology departments and other agencies specializing in cancer, use ICD-O.